This invention relates to bolts used in the building industry serving to fasten guard rails to concrete slabs, e.g. apartment balcony slabs, and referred to as guard rail bolts.
These bolts traverse the shoes of these guard rails via holes provided to this end and are driven into the concrete drilled in advance. Each shoe is held against the slab by virtue of a nut screwed on to the threaded head of the bolt and mounted on the plate in which the holes in the shoe have been provided.
In order to fix a bolt, the concrete slab is drilled to a sufficient length, to the diameter of the bolt, opposite each hole in the shoe, the bolt is driven in by striking its head, then the nut is screwed on, thereby ensuring fastening of the shoe.
The bolts used are generally of the self-undercutting and self-expanding type, i.e. capable upon fixing, on the one hand, of enlarging in a conical manner the hole drilled in the concrete at the end of the bolt and, on the other hand, of being expanded there, by the forced deformation of expansion lugs by means of a cone, in such a manner that it remains in contact with the walls of the hole having the diameter achieved in this manner.
The aim of the undercut is to relieve the stresses in the concrete around the hole in which the bolt is anchored. As the concrete does not remain locally prestressed once the bolt has been fixed, it consequently does not have the tendency to spall, in particular at the edge of the slab.
Normally, once the slab has been drilled, the undercut is produced when the bolt is driven into the concrete by striking and/or rotating its threaded head via a sleeve provided with a carbide coating to this end and covering the bolt, then the tightening of the nut results in the expansion of the lugs over a cone rigidly connected to the bolt, on the one hand, as a result of the reduction in the size of the bolt due to the tightening and, on the other hand, as a result of the fact that the lugs are held at this depth by the sleeve.
This fixing or fastening method has several disadvantages:                the presence of the sleeve means that the concrete has to be drilled to a diameter greater than the diameter of the bolt, e.g. in the case of a bolt having a diameter M12 of the international metric system, it is necessary to drill a hole of 18;        if the holes in the shoe are not drilled to this diameter, but to the smaller diameter of the bolts, the method also means that the concrete has to be drilled not through the shoe put in place in order to serve as a drilling guide, but before the shoe is placed in position, which may lead to drilling centre distances aligned incorrectly with those of the holes in the shoe;        as the sleeve has a predetermined length, the bolts can only be used for one single driving depth;        the undercut formed by the driving-in operation prestresses the concrete around the bolt before the tightening of the nut and therefore before the expansion of the bolt, as a result of which the mounting of the shoe of the guard rail on the slab cannot be controlled properly.        